1. Field of the Invention
This invention relates generally to the production of bar and rod products in continuous rolling mills by hot rolling discrete billets through roughing, intermediate and finishing mill sections, and is concerned in particular with a method and apparatus for the end-to-end welding of separate product lengths at a selected location along the rolling line, thereby making it possible to continue rolling uninterruptedly throughout the remainder of the mill.
2. Description of the Prior Art
In conventional bar and rod rolling mills currently in operation, the accepted practice is to begin the process with individual billets that are heated in a furnace and then discharged for separate rolling into product lengths having reduced cross sectional areas and front and tail ends. A number of drawbacks are associated with this practice. For example, the gap time between billets can account for as much as 10% lost production time.
Also, the front and tail ends of each product length are dimensionally and/or metallurgically unacceptable and must therefore be trimmed and discarded, resulting in considerable scrap losses. The front ends are also prone to cobbling, particularly at the higher speeds reached in the finishing section of the mill.
In an effort at avoiding these problems, technology has been developed to weld the billets end-to-end before they are introduced in the mill. However, this too has proven to be somewhat problematical. Welding of the large billet cross sections requires massive equipment and a heavy consumption of electrical energy. Excessive electrical energy is additionally required to inductively reheat the billet segments that have been chilled by contact with the heavy clamping devices required to force the billet ends together.
A further drawback with billet welding stems from the fact that the fused metal at the welded joint is subsequently rolled into a substantial length of the finished product. For example, when billets having a typical cross sectional area of 22,500 mm2 are welded and then rolled into 5.5 mm rod, the fused metal of the welded joint will extend through approximately 94 meters of the finished product. If the billets are metallurgically dissimilar, this length may well have to be scrapped, at a considerable loss to the mill operator.
The present invention applies welding technology to the rolling process in a manner and at a location along the rolling line that avoids or at least significantly minimizes the problems and disadvantages associated with current practices.